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Daytime dependence

In this topic we will introduce the different acquisition strategies, that exist to monitor land surfaces across the globe. At first we will take a look at optical remote sensing techniques that make use of the radiation originating from the sun.
Generally, passive instruments do not send pulses at any wavelength themselves, but solely rely on naturally existent radiation. assive sensors can only acquire data during daytime. Atmospheric effects that affect the amount of daylight that reaches ground, also impact the quality of data acquired by passive sensors.

Conceptual operating principle of passive remote sensing instruments

In the video below, the concept of optical remote sensing systems is explained.

Concept of optical remote sensing explained by Dr. Harm Bartholomeus

Passive microwave sensors

While commonly used optical remote sensing sensors are known to work passively, there are also microwave (or ‘radar’) instruments which do not provide their own source of illumination. Most radar sensors are able to produce interpretable data independent from time of day, not so with passive microwave sensors, which use the naturally available microwave radiation.

The underlying assumption on which passive microwave remote sensing is built, is that every object that has a finite temperature is constantly radiating energy. This idea has to be true to fit the principle of the thermal equilibrium.

As visualized in the figure above there a a number of key sources of passive microwave signals, that we can measure: (1) atmospheric emissions, (2) object emission (depending on surface temperature of a given object), (3) surface-reflected component and (4) transmitted subsurface component.

List of passive instruments

Optical sensors

Radar sensors